imho, the best HP mod for a 2.5 Boxster is learning how to drive it really, really well. My boxster (a 2.7) did not have the model designation on the rear trunk lid. Because of that, folks at DE/AX assumed it must be an "S" because I was "too fast" to be driving a base model. Yes. Learning how to drive well is a fantastic feeling because others will realize that it wasn't because of HP that you were fast. It was because of skill.
Learning to drive a 2.5 fast will make you even that much faster when the 3.4 goes in.
Enjoy!
As for mods that would carry over when the 3.4 is installed - bigger brakes and suspension would be where I would spend the $$.